Automated, Dynamic Minimization of Inter-cell Site Interference in CDMA Networks

ABSTRACT

Methods and apparatus for allocating scrambling codes to cells of a wireless network. In an example method, current scrambling code allocation information for a plurality of cells and network configuration information for a radio access network are received. A reallocation of scrambling codes to the plurality of cells is computed, based on the current scrambling code allocation information and the network configuration information, using a metaheuristic algorithm. A change in scrambling code for at least one of the plurality of cells is then triggered, based on the computed reallocation. In some embodiments, the metaheuristic algorithm is based on an objective function that comprises a summation of interference metrics for each of the plurality of cells, wherein the interference metrics depend on scrambling code allocations to the plurality of cells. In some embodiments, a simulated annealing metaheuristic is used.

TECHNICAL FIELD

The present invention generally relates to wireless communicationnetworks, and more particularly to the allocation of scrambling codesamong cells of a wireless communication network that uses code-divisionmultiple access (CDMA).

BACKGROUND

In the Wideband Code-Division Multiple Access (W-CDMA) networksdeveloped by members of the 3rd-Generation Partnership Project (3GPP),so-called scrambling codes are used to differentiate between downlinksignals transmitted by neighboring cells in the network, as well as todifferentiate between uplink signals transmitted to the cells by mobilestations in a given area. Generally, these scrambling codes are assignedto the cells in the network by the network operator, using any ofvarious cell planning tools.

Scrambling codes are also known as pseudo-noise codes and are one of twospreading codes groups used in W-CDMA systems. The other type of codeused in W-CDMA system is the channelization code, which is used forchannel separation of one transmission from another. Coding ofsubscriber information is achieved by “multiplying” the transmittedinformation with channelization and scrambling codes. More particularly,after the channelization codes is applied to user data to map the userdata to a CDMA channel, the data stream is multiplied by a code from agroup of special binary codes, to distinguish between differenttransmitters, which are in turn mapped to cells. The code gives a uniqueuser equipment (UE)/base station (BS) identity. This process is referredto as “scrambling” and the codes used for this process are hence called“scrambling codes.” The codes used are selected to produce a lowcorrelation value when correlated with other codes, which provides agood separation between multiple transmission sources.

In a basic W-CDMA network, since all transmitters are on the samefrequency there is no need for frequency planning. However, adequatephysical separation is required between cells that are using the samescrambling codes. There are 512 unique scrambling codes used in W-CDMA.Hence there is a need to maintain uniqueness of scrambling codes betweenadjacent W-CDMA cells.

W-CDMA handover decisions are taken by a Radio Network Controller (RNC)based on radio measurement data obtained by the user equipment (UE-3GPPterminology for a mobile terminal or access device) and reported to thenetwork. These measurements are performed to determine the quality(e.g., signal strength) of transmissions from the cell or cells that areserving the UE, as well as of transmissions from nearby cells. The RNCkeeps track of neighbor relations between the various cells managed bythe RNC; these configured neighbor definitions in the RNC are used toinform the UE of which scrambling codes must be measured. The3GPP-defined message containing a measurement order from the RNC to theUE has room for 32 IAF (intra-frequency) cells, including the active setcells. The ability for the RNC to transfer neighbor relation informationis limited to this number of neighbors. This is an important restrictionthat needs to be taken into consideration when planning the IAF neighborrelations.

3GPP specifications for W-CDMA operation also require the UE to findother strong cells apart from the ones requested by the RNC. However,the performance requirements for these measurements are less strict incomparison with what is required for the IAF monitored subset.

The RNC and UE communicate cell identities through scrambling codes.Since scrambling codes are re-used throughout a network, it is not aunique identifier. With the configured neighbor definitions, the RNC isable to uniquely identify a cell by verifying that the reportedscrambling code is in the list of neighbors. The configured neighborrelations are used to identify the scrambling codes the UE shouldmeasure among the 512 possible, when looking for handover candidates indedicated mode and cell selection/reselection in idle mode.

The scrambling code assigned to each cell must therefore be unique withrespect to scrambling codes assigned to other cells having an adjoiningboundary with the cell. These codes must be unique to avoid collisionswith the neighboring cell's downlink signals.

In current W-CDMA system implementations, the scrambling codes areusually assigned through a manual process by the network operator, usingcell planning tools that group and partition the scrambling codes andcluster these groups for macro- and micro-base station deployments.These scrambling code groups can then be assigned on a per cell sitebasis, to ensure both scrambling code uniqueness and an ability tore-use these groups efficiently.

Since the allocation of the scrambling code is a manual procedure, it issubject to human errors. A failure to maintain unique scrambling codesbetween adjacent cells could lead to false preamble detection for mobilestations from adjacent cells, and increase inter-cell site interference.Increased inter-cell site interference in turn can lead to a reductionin throughput and/or connectivity issues with mobile stations. In adeployment with thousands of base stations, it is not difficult toenvisage a situation where human error may lead to cell site planningissues. Hence, there is a need for improved techniques for ensuring anefficient allocation of the scrambling codes between cell sites, whilereducing or eliminating the need for operator intervention.

SUMMARY

Methods and apparatus for allocating scrambling codes to cells of awireless network are detailed herein. In one example method, currentscrambling code allocation information for a plurality of cells andnetwork configuration information for a radio access network arereceived. A reallocation of scrambling codes to the plurality of cellsis computed, based on the current scrambling code allocation informationand the network configuration information, using a metaheuristicalgorithm. A change in scrambling code for at least one of the pluralityof cells is then triggered, based on the computed reallocation. In someembodiments, the metaheuristic algorithm is based on an objectivefunction that comprises a summation of interference metrics for each ofthe plurality of cells, wherein the interference metrics depend onscrambling code allocations to the plurality of cells. In someembodiments, a simulated annealing metaheuristic is used.

Related methods for detecting and correcting problems with scramblingcode allocations among cells supported by a group of base stations inthe wireless network are also detailed herein. In an example of suchmethods, an initial one of the group of base stations is designated asource base station. In some embodiments, the designated base station isone of those base stations affected by a change in scrambling codeallocations. A first set of scrambling codes is then identified, thefirst set of scrambling codes consisting of all scrambling codesallocated to cells supported by the source base station. In embodimentstriggered by a reallocation of scrambling codes, the identified set ofscrambling codes reflects the one or more changes to be made to thecurrent scrambling code allocation. A second set of scrambling codes isdetermined, the second set of scrambling codes comprising at least allscrambling codes allocated to cells neighboring any of the cellssupported by the source base station.

Next, the first and second sets of scrambling codes are compared, todetect duplicate scrambling codes between the first and second sets.Upon detection of a duplicated scrambling code between the first andsecond sets, location information for the cells corresponding to theduplicated scrambling code is used to determine whether interferencebetween the cells is likely and, if interference is likely, thescrambling code is changed for the cell that has the duplicatedscrambling code and that is supported by a base station other than thesource base station. A next one of the base stations is selected fordesignation as the source base station, and the identifying,determining, comparing, using, changing, and selecting operationssummarized above are repeated until each one of the base stations hasbeen designated as the source base station.

Network node apparatus adapted to carry out any of the severaltechniques summarized above, and variants thereof, are also disclosed inthe detailed discussion that follows. Of course, the present inventionis not limited to the above-summarized features and advantages. Indeed,those skilled in the art will recognize additional features andadvantages upon reading the following detailed description, and uponviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference numerals designate corresponding similarparts, operations, or system components. The features of the variousillustrated embodiments can be combined unless they exclude each other.Embodiments of the presently disclosed methods and apparatus aredepicted in the drawings and are detailed in the description thatfollows.

FIG. 1 is a schematic diagram of a W-CDMA network include a networkmanagement according to embodiments of the presently disclosed methodsand apparatus.

FIG. 2 is a block diagram illustrating features of an example networkmanagement node according to the presently disclosed techniques andapparatus.

FIG. 3 is a process flow diagram showing an example method forreallocating scrambling codes according to some embodiments of thepresently disclosed techniques.

FIG. 4 is a process flow diagram illustrating an example implementationof a metaheuristic algorithm for reallocating scrambling codes.

FIG. 5 is a process flow diagram illustrating an example method fordetecting and correcting problems in scrambling code allocations,according to some embodiments of the presently disclosed techniques.

FIG. 6 is a block diagram illustrating another representation of anetwork management node, according to some embodiments.

FIG. 7 is a block diagram illustrating still another representation of anetwork management node, according to some embodiments.

DETAILED DESCRIPTION

Within the context of this disclosure, the terms “mobile station,”“mobile terminal,” “wireless terminal,” or “wireless device” refer toany terminal that is able to communicate wirelessly with an access nodeof a wireless network by transmitting and/or receiving wireless signals.Thus, the term “mobile station,” for example, encompasses, but is notlimited to: a user equipment (UE), as that term is used in 3GPPspecifications for W-CDMA and other networks, whether that userequipment is a cellular telephone, smartphone, wireless-equipped tabletcomputer, etc.; a stationary or mobile wireless device for so-calledmachine-to-machine (M2M) communication or machine-type communication(MTC); or an integrated or embedded wireless card forming part of acomputer or other electronic equipment; a wireless card, dongle, or thelike, for plugging in to a computer or other electronic equipment.Throughout this disclosure, the terms “user equipment” and “UE” aresometimes used to exemplify various embodiments. However, this shouldnot be construed as limiting, as the concepts illustrated herein areequally applicable to other wireless terminals. Hence, whenever a “userequipment” or “UE” is referred to in this disclosure, this should beunderstood as encompassing any mobile terminal or wireless terminal asdefined above. Likewise, the terms “base station,” “NodeB,” “evolvedNodeB,” “eNB,” “radio base station,” or the like are used to refer to anaccess point of a wireless communication network, which communicateswith one or more mobile stations via radio communications.

In the discussion that follows, specific details of particularembodiments of the presently disclosed techniques and apparatus are setforth for purposes of explanation and not limitation. It will beappreciated by those skilled in the art that other embodiments may beemployed apart from these specific details. Furthermore, in someinstances detailed descriptions of well-known methods, nodes,interfaces, circuits, and devices are omitted so as not to obscure thedescription with unnecessary detail. Those skilled in the art willappreciate that the functions described may be implemented in one or inseveral nodes.

Some or all of the functions described may be implemented using hardwarecircuitry, such as analog and/or discrete logic gates interconnected toperform a specialized function, ASICs, PLAs, etc. Likewise, some or allof the functions may be implemented using software programs and data inconjunction with one or more digital microprocessors or general purposecomputers. Where nodes that communicate using the air interface aredescribed, it will be appreciated that those nodes also have suitableradio communications circuitry. Moreover, the technology canadditionally be considered to be embodied entirely within any form ofcomputer-readable memory, including non-transitory embodiments such assolid-state memory, magnetic disk, or optical disk containing anappropriate set of computer instructions that would cause a processor tocarry out the techniques described herein.

Hardware implementations may include or encompass, without limitation,digital signal processor (DSP) hardware, a reduced instruction setprocessor, hardware (e.g., digital or analog) circuitry including butnot limited to application specific integrated circuit(s) (ASIC) and/orfield programmable gate array(s) (FPGA(s)), and (where appropriate)state machines capable of performing such functions.

In terms of computer implementation, a computer is generally understoodto comprise one or more processors or one or more controllers, and theterms computer, processor, and controller may be employedinterchangeably. When provided by a computer, processor, or controller,the functions may be provided by a single dedicated computer orprocessor or controller, by a single shared computer or processor orcontroller, or by a plurality of individual computers or processors orcontrollers, some of which may be shared or distributed. Moreover, theterms “processor” and “controller” may also refer to other hardwarecapable of performing such functions and/or of executing software, suchas the example hardware recited above.

References throughout the specification to “one embodiment” or “anembodiment” mean that a particular feature, structure, or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the present invention. Thus, the appearance of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthe specification are not necessarily all referring to the sameembodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

While the following examples are described in the context of W-CDMAsystems, the principles described in the following disclosure may beequally applied to other functional contexts and other cellular networksthat use scrambling codes.

FIG. 1 is a simplified schematic of a radio communications network, suchas a WCDMA network, or similar. The radio communications networkcomprises a radio network node, e.g. a first radio base station 12,providing radio coverage over at least one geographical area forming acell, a first cell 11. The definition of a cell may also incorporate thefrequency band used for transmissions, which means that two or moredifferent cells may cover the same geographical area, but usingdifferent frequency bands. A first user equipment 10 is served in thefirst cell 11 by the first radio base station 12 and may becommunicating with the first radio base station 12. The first userequipment 10 transmits data over an air or radio interface to the firstradio base station 12 in uplink (UL) transmissions and the first radiobase station 12 transmits data over an air or radio interface to thefirst user equipment 10 in downlink (DL) transmissions. The illustratedradio communications network further comprises a second radio basestation 13 controlling a second cell 14, which is serving a second userequipment 15. The first radio base station 12 and the second radio basestation 13 are controlled by a controlling radio network node,illustrated as a Radio Network Controller (RNC) 16. A wireless networkmay include many RNCs, each controlling several base stations.

Also shown in FIG. 1 is a network management node 18, which is acomputerized node that is connected to each of a plurality of RNCs inthe wireless communication network, including RNC 16. Network managementnode 18 provides the system operator access to each of the RNCs for thepurposes of system configuration and management. Among other things,network management node 18 is configured to direct the allocation ofscrambling codes to base stations that are under the control of theRNCs. As described in further detail below, network management node 18in some embodiments is configured to obtain scrambling code allocationinformation, neighbor cell information, and other information from eachRNC, and to analyze the information to determine whether scrambling codeallocations should be changed. If changes are necessary, networkmanagement node 18 communicates those changes to the RNCs, which in turncommunicate the changes to the base stations under their control.

FIG. 2 illustrates details of an example network management node 18 towhich, in which, and with which the presently disclosed techniques maybe applied. In an example configuration, the network management node 18is a computerized platform having a network interface circuit 28 thatallows system management node 18 to communicate with RNCs or similarnetwork nodes in a wireless communication network, so that configurationinformation can be exchanged between system management node 18 and thoseRNCs or other network nodes. Network management node 18 may be astand-alone platform, in some embodiments, or may be combined with othernetwork management functionalities. In some embodiments, networkmanagement node 18 may comprise user interface circuitry 27, allowingfor direct interaction with network management node 18. This userinterface circuitry 27 may comprise conventional displays, keyboards,and/or other input and output devices. In other embodiments, operatorinteraction with network management node 18 may be limited tointeraction through remotely connected computers, connected to systemmanagement node 18 through the network interface circuit 28 which may bearranged to support, for example, IP-based data communications over anyof one or more wired or wireless network interfaces, such as an Ethernetand/or wireless local-area network (WLAN) interface. Some embodiments ofnetwork management node 18 may support both remote interaction (throughnetwork interface 28) as well as local interaction (through userinterface hardware 27).

In the illustrated example, network management node 18 includes aprocessing circuit 20, which in turn includes associated memory/storage22. The memory/storage 16 may be one or more types of computer-readablemedium, such as a mix of volatile, working memory and non-volatileconfiguration and program memory or storage. In the example shown inFIG. 2, program data 24 and other data 26 are stored in memory 22. Insome embodiments, program data 24 comprises computer program instructionis that, when executed by processing circuit 20, cause the processingcircuit to carry out one or more of the several techniques describedherein for allocating scrambling codes and/or adjusting the allocationof scrambling codes in a wireless network. Of course, processing circuit20 is further configured, e.g., with appropriate program instructionsand/or with appropriate hardware, to control, send data to, and receivedata from, user interface hardware 27 and network interface 28.

While FIGS. 1 and 2 illustrate a stand-alone network management node 18that communicates with one or more radio network controllers 16 and thatis configured to carry out one or more of the techniques detailed hereinfor reallocating scrambling codes and/or for detecting and correctingproblems with scrambling code allocations, the same techniques may beimplemented in one or more existing network nodes, in some embodiments.For example, an existing radio network controller (RNC) design may beadapted so that the RNC is configured to carry out one or more of thetechniques described herein in addition to carrying out its normal RNCfunctions. It will be appreciated that the block diagram of FIG. 2applies to such cases as well, but that the processing circuit 20 insuch cases may be further configured to carry out network operationsthat are generally unrelated to the techniques described herein.

As noted above, in current W-CDMA system implementations, the scramblingcodes are usually assigned through a manual process by the networkoperator. Since the allocation of the scrambling code is a manualprocedure, the process is open to human errors. If scrambling codes arenot unique between adjacent cells, false preamble detections by mobilesfrom adjacent cells may occur, and inter-cell site interference isincreased. Increased inter-cell site interference can lead to areduction in throughput and/or connectivity issues with UEs.

In a deployment with thousands of base stations, it is not difficult toenvisage a situation where human error may lead to cell site planningissues. Hence, techniques for scrambling code allocation that willreduce or eliminate the need for operator intervention while ensuringefficient allocation of the scrambling code between cell sites areneeded. In addition, techniques for detecting and correcting problemswith duplicated scrambling are needed.

Existing solutions based either on manual assignments of scramblingcodes or semi-automated techniques for assigning scrambling codes havecritical drawbacks. First, even if quality algorithms are used toautomatically assign the scrambling codes during initial site planning,the allocations are still prone to future changes by operators. Sincethe scrambling code is adjustable on a per-cell basis, it is possiblethat an operator may modify the scrambling code during a subsequent sitemaintenance activity. This can lead to errors in allocating thescrambling code. Re-running the algorithm would lead to a longer sitemaintenance window and affect service for users already being servedunder W-CDMA cells.

If the allocations are manually planned, the scrambling code sequence issubject to human error. For example, a database must be maintained withscrambling code entries for each cell. Any mistakes in allocating thescrambling code will lead to inter-cell site interference, if the cellsare adjacent to each other.

Therefore an automated and periodic approach to allocating thescrambling codes is required, since these codes must be unique betweencells to ensure no impact to UE mobility and activity, and since theallocations must reflect changes in network configuration over time.

Accordingly, embodiments of the present invention include methods ofautomatically allocating scrambling codes by a network manager node thatis used to administer the cells in a given portion or all of a radioaccess network (RAN). The network manager according to these embodimentswill periodically traverse the configured cell sites and check to ensurethat there is no-overlap of the scrambling code sequence betweenadjacent cells. Adjacent cells are determined by the Automatic neighborrelations (ANR) configured on the RNC, as well as cell site locationinformation available from the network plan in the network manager. Cellsite location information helps ensure that the scrambling code isunique between adjacent cells, in the event that the ANR entries areincorrectly configured by the network operator. Cell radius is comparedbetween cell sites configured amongst the cells in the network plan, toensure that any overlapping cells have unique scrambling codes.

According to some embodiments of this approach, a network manager nodeoperatively connected to a RAN carries out a process for allocatingscrambling codes to cells in the RAN. An example of such a process isshown in FIG. 3.

As shown at block 310, the network manager node receives networkconfiguration information that defines the configuration of RNCs andcell sites in the network. This information may be received viaconfiguration information input by operator personnel, and/or may bereceived and/or updated via information received directly from theinvolved RNCs or other network nodes. The network manager node alsoreceives current scrambling code allocation for the cells in the networkor in the portion of the network to be managed—again, this informationmay be received from the RNCs, for example.

Using this information, the network manager node parses through the RNCsconfigured in the cell site plan and through the cells managed by thenode and computes a reallocation of scrambling codes to cells, using ametaheuristic algorithm. This is shown at block 320. As shown at block330, the network manager node then triggers any changes in scramblingcodes that are needed, based on the computed reallocation. Thistriggering may involve sending reconfiguration messages to one or moreaffected RNCs, for example, using existing 3GPP-defined interfaces orother communications interfaces to the RNCs.

Note that the scrambling code reallocation methods described herein canbe implemented as a feature within the network manager, and performed ona regular or periodic basis, or can be invoked during site planning.Since the end result is the configuration of RNCs in such a way thatduplication of scrambling codes is avoided, the implementation isflexible.

In some embodiments of the process shown in FIG. 3, after thereallocation has been computed, the computed reallocation is tested toensure that that there are no duplicated scrambling codes amongneighboring cells. Specific techniques for carrying out this test aredescribed in detail below, in connection with FIG. 5.

In some embodiments, neighbor cells for at least some of the pluralityof cells are determined based at least in part on neighbor cellinformation obtained from one or more radio network controllers (RNCs).In some of these and in some other embodiments, neighbor cells aredetermined for at least some of the plurality of cells based at least inpart on geo-positioning information for at least some of the pluralityof cells.

In some embodiments, at least the computing operation illustrated atblock 320 is repeated at pre-determined intervals. In this manner, thereallocation of scrambling codes can be carried out as a backgroundmaintenance task. In other embodiments or in some instances, thecomputing and triggering operations shown in blocks 320 and 330 areinitiated in response to a problem detection in the RAN. Similarly, thecomputing and triggering operations shown in blocks 320 and 330 may beinitiated in response to an addition of a new cell to the RAN, in someembodiments or in some instances.

In some embodiments, the metaheuristic algorithm used for computing thereallocation of scrambling codes is based on an objective function thatcomprises a summation of interference metrics for each of the pluralityof cells, where the interference metrics depend on scrambling codeallocations to the plurality of cells. The metaheuristic algorithm seeksto optimize the objective function over all possible states s of thenetwork, where each states reflects a possible allocation of scramblingcodes to all of the cells in the network or in the relevant portion ofthe network.

One example of such an objective function C(s) is given below:

C(s)=Σ_(i)RNC_(i)(Σ_(j)NodeB _(j)(Σ_(k)Cell_(k)ICI_(kji))))*y(s),

where ICI_(kji) represents the inter-cell interference (ICI) at the k-thcell of the j-th NodeB managed by the i-th RNC of the RAN. In this case,the objective is to minimize the total inter-cell interference, so theobjective of the metaheuristic algorithm is to minimize C(s). ICI_(kji)is estimated for a given allocation of scrambling codes that correspondto a system state s; in the expression above, y(s) is an integervariable that is equal to 1 if state s is chosen, and is 0 otherwise.Note that the estimation of the inter-cell interference may reflect anyof a variety of performance data (e.g., key-performance indicators, orKPIs) collected for the affected nodes, such as cell data traffic, thenumber of UEs attached to a cell and/or its neighbors in recentobservations, with neighboring cells impacting more. The estimate of theinter-cell interference may also reflect collected network configurationinformation—for instance, cell power levels, cell locations, antennaorientations, and the like may be considered. Also note that RNC_(i),NodeB_(j), and Cell_(k) are optional weights that allow the objectivefunction to reflect relative priorities among the RNCs, NodeBs, andcells of the network.

The formulation of the objective function may reflect all of the basestations in a RAN or only a portion of the RAN. The objective functioncould be targeted to optimization of a particular geographic market,such as a city, for example.

FIG. 4 illustrates an example metaheuristic algorithm as applied to thepresent problem of reallocating scrambling codes among cells in a radioaccess network. The illustrated algorithm employs the well-knownsimulated annealing metaheuristic, which employs a “temperature”-basedstrategy to avoid local minima. The basic idea of the simulatedannealing metaheuristic is to allow some changes in state that result inworse results than a current solution, to escape from local minima in asearch space with a complex shape. These changes can be regarded as“uphill” changes. The probability of such a change decreases over time,as the temperature of the process “cools.” Other metaheuristics may beused instead of or along with the simulated annealing metaheuristic, inother embodiments.

Referring to FIG. 4, it can be seen that the process begins with theinitial configuration s (i.e., the initial allocation scrambling codes).As shown at block 410, an initial temperature value T is selected orcalculated. As shown at block 420, an initial value C(s) of theobjective function is calculated. Note that in some embodiments, theprocess may start at an arbitrary predetermined temperature, while inothers the starting temperature may be calculated based on the initialvalue C(s) of the objective function. In the latter case, of course, theobjective function calculation shown at block 420 must be performedbefore the initial temperature T is obtained.

As shown at block 430, the process continues with moving to a randomstate s′, by changing one or more scrambling code allocations from theprevious state, and calculating the value of the randomly selected states′, i.e., C(s′). In some embodiments, a predetermined number ofscrambling codes are changed at each iteration; in some embodiments onlya single scrambling code is changed at each iteration. The particularscrambling code to be changed can be selected completely randomly fromamong all the cells in the area subject to optimization, in someembodiments. In others, a scrambling code to be changed may be selectedat random from those corresponding to cells at one or a fewpredetermined base stations, or from those cells managed by a particularRNC. In some embodiments, a particular base station or RNC to be focusedon may be identified in response to a detected network performanceproblem at or near the selected nodes, for example.

In some embodiments, possible scrambling code changes are tested beforethe process continues, to ensure that the change or changes do notresult in any duplications among the modified cell and its neighbors. Inother words, the selection of a random state s′ is constrained so as toavoid duplicate scrambling codes among neighbors. In other embodiments,concerns about duplicate scrambling codes are handled later.

As shown at block 440, the objective function value C(s′) for thecandidate state s′ is compared to the objective function value C(s) forthe previous state, which in the first iteration will be the initialstate. If C(s′) is better than C(s), the candidate state s′ is“accepted,” and becomes the new current state s. Note that determiningwhether C(s′) is “better” than C(s) involves simply determining whetherC(s′) is lower than C(s) in embodiments where the objective function isformulated so that it should be minimized. It will be appreciated thatit is possible to formulate an objective function that should bemaximized to improve system performance; in such embodiments, oneobjective function value is “better” value than another when it ishigher.

As seen at block 450, in some instances a candidate s′ will be acceptedas the new current state even if the resulting objective function valueC(s′) is not better than the previous value. This accepting of adegraded state is performed randomly, using an acceptance probabilityfunction P(s, s′, T), that depends on the objective function values C(s)and C(s′), as well as on the temperature T, such that the likelihood ofaccepting a worse state s′ decreases as the temperature decreases. Theprobability function might be, for example:

${{P\left( {s,s^{\prime},T} \right)} = {\exp \left( \frac{{C(s)} - {C\left( s^{\prime} \right)}}{T} \right)}},$

in some embodiments. As seen at block 450, a randomly generated valueRAND is compared to P(s, s′, T); if RAND is less than P(s, s′, T), thenstate s′ is accepted, otherwise it is rejected. Each accepted state issaved, in some embodiments, for later evaluation.

Whether or not the candidate state s′ is accepted as the current state,an iteration counter is updated and compared to a maximum number ofiterations, as shown at block 460. If the maximum number of iterationsis reached, the process concludes, as shown at block 470. Otherwise, thetemperature is reduced, as appropriate, as shown at block 480, and theoperations shown at blocks 430, 440, 450, and 460 are repeated until themaximum number of iterations is reached. In the illustrated process, thetemperature T is reduced by 5% once every four-hundred iterations; otherschedules for reducing the temperature T may be used.

At the conclusion of the iterative process shown in FIG. 4, the final“current” state s may be used as the optimal state, in some embodiments,and the necessary changes to scrambling codes determined by comparingthis final state to the initial state. In other embodiments, the saved“accepted” states are evaluated, to identify an optimal state from amongthe accepted states. For example, the state configuration that willtraverse the least number of base stations, i.e., requiring the fewestscrambling code changes, while minimizing the network's objectivefunction C(s) may be chosen. The selection of the optimal state from thesaved states may reflect a balancing between the number of scramblingcode changes that are needed against the expected improvements to theobjective function, so that complex system changes that will result inonly minimal improvements to the objective function are avoided. Forexample, given one saved state that requires n changes and another thatrequires more than n+i changes, the latter state may be adopted as theoptimal state only in the event that the expected improvement in theobjective function exceeds a predetermined threshold X. This threshold Xmay be a function of the incremental scrambling code changes i, in someembodiments.

FIG. 5 illustrates an example method for implementation in a networknode operatively connected to a wireless network, for detecting andcorrecting problems with scrambling code allocations among cellssupported by a group of base stations in the wireless network, whereeach base station supports one or more of the cells. As discussed above,this process may be implemented in a network management node operativelyconnected to one or more base stations in the wireless network or to oneor more radio network controllers (RNCs) in the wireless network, insome embodiments. In others, the process may be carried out in an RNC,or in some other network node. The illustrated process may be carriedout after a scrambling code reallocation has been performed, e.g., aftera process like that illustrated in FIGS. 3 and/or 4 has been carriedout. In some embodiments or instances, the illustrated process may becarried out in response to a detected performance problem in thenetwork, or in response to determining that one or more cells have beenadded to or subtracted from the network, or re-configured.

As shown at block 510, the illustrated process begins with designatingan initial one of the group of base stations as a source base station.Next, as shown at block 520, a first set of scrambling codes isidentified, the first set of scrambling codes consisting of allscrambling codes allocated to cells supported by the source basestation. In addition, as shown at block 530, a second set of scramblingcodes is determined, the second set of scrambling codes comprising atleast all scrambling codes allocated to cells neighboring any of thecells supported by the source base station. In some embodiments,determining the second set of scrambling codes comprises identifyingscrambling codes allocated to cells neighboring any of the cellssupported by the source base station using system-configured neighborrelations or using location data corresponding to the cells, or both.

In an important variant, the second set of scrambling codes may includetwo subsets: a first subset comprising all scrambling codes allocated tocells neighboring any of the cells supported by the source base stationand a second subset including or more scrambling codes identified asclosely related to one or more of the scrambling codes in the firstsubset, wherein scrambling codes are identified as closely related basedon predetermined relationships between scrambling codes. Thesepredetermined relationships may reflect a priori knowledge of code pairsthat are likely to cause inter-cell interference when used in adjacentcells; this a priori knowledge may be mathematically derived, e.g.,through simulations, or through empirical observation of inter-cellinterference.

As shown at block 540, the first and second sets of scrambling codes arecompared, to detect duplicate scrambling codes between the first andsecond sets. Upon the detecting of a duplicated scrambling code betweenthe first and second sets, location information for the cellcorresponding to the duplicated scrambling code is used to determinewhether interference between the cells is likely, as shown at block 550.In some embodiments, this may comprise comparing a distance between thecells to a cell radius for one or both of the cells to determine whetherinterference is likely. GPS information identifying the base stationlocations may be used, for example. In some embodiments, antennaorientation information for the cells corresponding to the duplicatedscrambling codes may be used to determine whether interference betweenthe cells is likely. If interference is likely, e.g., because theneighboring cells are located too close together and/or have conflictingantenna orientations, the scrambling code is changed for the cell thathas the duplicated scrambling code and that is supported by a basestation other than the source base station, as shown at block 560.Changing the scrambling code may require sending a reconfigurationmessage to the affected RNC, for example. This message may be sent overa 3GPP-defined interface (e.g., the Iur interface) or some othercommunications interface.

As shown at block 570, the process continues with the selection of anext one of the base stations for designation as the source basestation. The identifying, determining, comparing, using, changing, andselecting operations shown in blocks 520-570 are repeated until each oneof the base stations has been designated as the source base station.

In some embodiments, the process shown in FIG. 5 may incorporate one ormore planned changes to a current scrambling code allocation among thecells. For example, the process shown in FIG. 5 may be carried out aftera reallocation of scrambling codes carried out according to theprocesses shown in FIGS. 3 and/or 4, in which case the changes to thescrambling code allocation are an input to the process of FIG. 5. Inthis case, the designation of the initial one of the base stations as asource base station may be based on the planned changes to thescrambling code reallocation. Thus, one of those base stations affectedby the scrambling code changes may be designated as the initial sourcebase station—in some cases, the base station corresponding to thescrambling code change that is expected to result in the biggest overallchange to inter-cell interference may be selected as the initial basestation. This may be determined for example, by observing the systemstate change that resulted in the biggest change to the objectivefunction used in the scrambling code reallocation process. In suchembodiments, the identified scrambling codes corresponding to the sourcebase station at each iteration should reflect the one or more changes tobe made to the current scrambling code allocation. In subsequentiterations, other base stations affected by the scrambling codereallocation may be selected as source base stations, prior totraversing other base stations in the network.

The processes shown in FIGS. 3, 4, and 5 may be performed on an entirenetwork or only on a portion of a network. For example, one of theprocesses above may be performed only on a localized portion of anetwork when new neighboring cells are added to a particular basestations, or when the process is triggered by a performance degradationin a particular area of the network.

As previously mentioned, a network node having a configuration like thatshown in FIG. 2 may be used to carry out one or more of the severalmethods detailed above. This may be a stand-alone network managementnode, a modified radio network node, or some other node in or attachedto the wireless network node. Accordingly, it should be understood thatembodiments of the presently disclosed techniques and apparatus includea network management node that comprises communications interfacecircuitry configured to communicate with one or more nodes in a radioaccess network (RAN), and a processing circuit configured to controlcommunications interface circuitry and to carry out a method like thatshown in FIG. 3, i.e., to: receive current scrambling code allocationinformation for the plurality of cells and receive network configurationinformation for the radio access network; compute a reallocation ofscrambling codes to the plurality of cells, based on the currentscrambling code allocation information and the network configurationinformation, using a metaheuristic algorithm; and trigger a change inscrambling code for at least one of the plurality of cells, based on thecomputed reallocation. Likewise, other embodiments include a networknode, comprising communications interface circuitry configured tocommunicate with one or more nodes in a radio access network (RAN) and aprocessing circuit configured to control the communications interfacecircuitry and to carry out a method like that shown in FIG. 5, i.e., to:designate an initial one of the group of base stations as a source basestation; identify a first set of scrambling codes, the first set ofscrambling codes consisting of all scrambling codes allocated to cellssupported by the source base station; determine a second set ofscrambling codes, the second set of scrambling codes comprising at leastall scrambling codes allocated to cells neighboring any of the cellssupported by the source base station; compare the first and second setsof scrambling codes to detect duplicate scrambling codes between thefirst and second sets; upon detecting a duplicate scrambling codebetween the first and second sets, use location information for thecells corresponding to the duplicated scrambling code to determinewhether interference between the cells is likely and, if interference islikely, change the scrambling code for the cell that has the duplicatedscrambling code and that is supported by a base station other than thesource base station; select a next one of the base stations fordesignation as the source base station; and repeat the identifying,determining, comparing, using, changing, and selecting operations untileach one of the base stations has been designated as the source basestation.

Furthermore, it will be appreciated that the network management node 18illustrated in FIG. 2, and in particular the processing circuit 20therein, may be conceived as comprising several functional units, any oreach of which may be implemented using one or more processing elementsconfigured with appropriate software, firmware, and/or supportingdigital hardware. Thus, FIG. 8 illustrates another representation ofnetwork management node 18, which in this case is adapted to carry out atechnique like that shown in FIG. 3. Network management node 18 in thiscase comprising network interface circuitry 28, (optional) userinterface hardware 27, and several functional units. These functionalunits include, in addition to the communications interface circuitry 28,which is configured to communicate with one or more network nodes in aRAN, a receiving unit 62 for receiving, e.g., via the network interfacecircuit 28, current scrambling code allocation information for theplurality of cells and receiving network configuration information forthe radio access network. Also included is a computing unit 64 forcomputing a reallocation of scrambling codes to the plurality of cells,based on the current scrambling code allocation information and thenetwork configuration information, using a metaheuristic algorithm, anda triggering unit 66 for triggering a change in scrambling code for atleast one of the plurality of cells, based on the computed reallocation.

FIG. 9 illustrates another representation of network management node 18,in this case adapted to carry out a technique like that shown in FIG. 5.According to this representation, network management node 18 againincludes network interface circuitry 28, (optional) user interfacehardware 27, and several functional units. The functional units includea designating unit 72 for designating an initial one of the group ofbase stations as a source base station, an identifying unit 74 foridentifying a first set of scrambling codes, the first set of scramblingcodes consisting of all scrambling codes allocated to cells supported bythe source base station, and for determining a second set of scramblingcodes, the second set of scrambling codes comprising at least allscrambling codes allocated to cells neighboring any of the cellssupported by the source base station. A comparing unit 76 is provided,for comparing the first and second sets of scrambling codes to detectduplicate scrambling codes between the first and second sets, and achanging unit 74, upon detecting a duplicated scrambling code betweenthe first and second sets, uses location information for the cellscorresponding to the duplicated scrambling code to determine whetherinterference between the cells is likely and, if interference is likely,changes the scrambling code for the cell that has the duplicatedscrambling code and that is supported by a base station other than thesource base station. Finally, a selecting unit is provided, forselecting a next one of the base stations for designation as the sourcebase station. The identifying unit 74, comparing unit 76, changing unit78, and selecting unit 80 are operated in an iterative fashion to carryout the method shown in FIG. 5.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiments withoutdeparting from the scope of the present invention. For example, it willbe readily appreciated that although the above embodiments are describedwith reference to parts of one or more 3GPP-based networks, anembodiment of the present invention will also be applicable to likenetworks, such as a successor of the 3GPP network, having likefunctional components. Therefore, in particular, the terms 3GPP andassociated or related terms used in the above description and in theenclosed drawings and any appended claims now or in the future are to beinterpreted accordingly.

Examples of several embodiments of the present invention have beendescribed in detail above, with reference to the attached illustrationsof specific embodiments. Because it is not possible, of course, todescribe every conceivable combination of components or techniques,those skilled in the art will appreciate that the present invention canbe implemented in other ways than those specifically set forth herein,without departing from essential characteristics of the invention. Thepresent embodiments are thus to be considered in all respects asillustrative and not restrictive.

What is claimed is:
 1. A method, in a network management nodeoperatively connected to a radio access network that comprises aplurality of cells, for allocating scrambling codes to the cells, themethod comprising: receiving current scrambling code allocationinformation for the plurality of cells and receiving networkconfiguration information for the radio access network; computing areallocation of scrambling codes to the plurality of cells, based on thecurrent scrambling code allocation information and the networkconfiguration information, using a metaheuristic algorithm; andtriggering a change in scrambling code for at least one of the pluralityof cells, based on the computed reallocation.
 2. The method of claim 1,wherein the metaheuristic algorithm is based on an objective functionthat comprises a summation of interference metrics for each of theplurality of cells, wherein the interference metrics depend onscrambling code allocations to the plurality of cells.
 3. The method ofclaim 1, wherein the metaheuristic algorithm employs a simulatedannealing metaheuristic.
 4. The method of claim 1, wherein apredetermined number of scrambling codes are changed from each iterationof the metaheuristic algorithm to the next.
 5. The method of claim 4,wherein the predetermined number is
 1. 6. The method of claim 1, whereinscrambling code changes from each iteration of the metaheuristicalgorithm to the next are tested to ensure that there are no duplicatescrambling codes among neighboring cells.
 7. The method of claim 6,further comprising determining neighbor cells for at least some of theplurality of cells based at least in part on neighbor cell informationobtained from one or more radio network controllers.
 8. The method ofclaim 6, further comprising determining neighbor cells for at least someof the plurality of cells based at least in part on geo-positioninginformation for at least some of the plurality of cells.
 9. The methodof claim 1, further comprising, after the reallocation has beencomputed, testing the computed reallocation to ensure that that thereare no duplicate scrambling codes among neighboring cells.
 10. Themethod of claim 1, wherein at least the computing operation of claim 1is repeated at pre-determined intervals.
 11. The method of claim 1,wherein at least the computing and triggering operations are initiatedin response to a problem detection in the radio access network.
 12. Themethod of claim 1, wherein at least the computing and triggeringoperations are initiated in response to an addition of a new cell to theradio access network.
 13. A network management node, comprising:communications interface circuitry configured to communicate with one ormore nodes in a radio access network, and a processing circuitconfigured to control the communications interface circuitry and to:receive current scrambling code allocation information for the pluralityof cells and receive network configuration information for the radioaccess network; compute a reallocation of scrambling codes to theplurality of cells, based on the current scrambling code allocationinformation and the network configuration information, using ametaheuristic algorithm; and trigger a change in scrambling code for atleast one of the plurality of cells, based on the computed reallocation.14. The network management node of claim 13, wherein the metaheuristicalgorithm is based on an objective function that comprises a summationof interference metrics for each of the plurality of cells, wherein theinterference metrics depend on scrambling code allocations to theplurality of cells.
 15. The network management node of claim 13, whereinthe metaheuristic algorithm employs a simulated annealing metaheuristic.16. The network management node of claim 13, wherein the processingcircuit is configured to change a predetermined number of scramblingcodes from each iteration of the metaheuristic algorithm to the next.17. The network management node of claim 13, wherein the processingcircuit is configured to test scrambling code changes from eachiteration of the metaheuristic algorithm to the next to ensure thatthere are no duplicate scrambling codes among neighboring cells.
 18. Thenetwork management node of claim 17, wherein the processing circuit isfurther configured to determine neighbor cells for at least some of theplurality of cells based at least in part on neighbor cell informationobtained from one or more radio network controllers (RNCs).
 19. Thenetwork management node of claim 17, wherein the processing circuit isfurther configured to determine neighbor cells for at least some of theplurality of cells based at least in part on geo-positioning informationfor at least some of the plurality of cells.
 20. The network managementnode of claim 13, wherein the processing circuit is further configuredto test, after the reallocation has been computed, the computedreallocation, to ensure that that there are no duplicate scramblingcodes among neighboring cells.